Uses of conductivity meter? - UK Vintage Radio Repair and Restoration Discussion Forum

Julesomega · 16th Aug 2020, 4:25 pm

I bought an interesting-looking meter off eBay under the same misapprehension as the seller, who listed it as a "Siemens meter"
It is indeed, but was made by WPA of Linton, Cambridge, model CM35 Conductivity Meter

It covers 10^-6 to 30^-2 Siemens. Not having any known conductors to hand I tried it with some resistors and it is reasonably accurate over this resistance range. My mind turns to jelly trying to convert between units, so I'm wondering who would want a meter to measure over this rather limited range instead of just picking up a multimeter?

It is rather nice that the meter doesn't deflect on O/C, but do you really need a four-terminal measurement to measure down to only 33Ω?

Sinewave · 16th Aug 2020, 4:44 pm

It's for meausring water/solution purity. Used in the process/plant industry. Last time I saw one was in the print industry for measuring purity for litho web offset press fount solution.

GMB · 16th Aug 2020, 5:06 pm

Do they measure with DC or AC?

I was just about to build one, or at least something rather like it.
I had assumed you had to use AC but if it is a 4-terminal device I guess that would be another way to go.

Radio Wrangler · 16th Aug 2020, 5:25 pm

Conductivity meters often go with some sort of test cell. Some are for measuring liquids/pastes, others for powders.

Some test cells ate flat surfaces for measuring the conductivity of antistatic surfaces.

Sometimes used in agriculture for measuring the water content of grains to determine how much drying is needed. These versions may have a scale intended for the purpose and the specific test cell.

David

DMcMahon · 16th Aug 2020, 5:33 pm

Used similar meters in de-ionised water heat exchangers that were used to cool High Voltage equipment.

Older non SI unit name for Siemens (Conductance) was the Mho.

ParcGwyn · 16th Aug 2020, 5:42 pm

They used to be used for checking moisture levels in wheat. the wheat was ground up and placed in cell that was compressed using a G-clamp. The more moisture the higher the conductivity and the lower the price. Not sure if your instrument falls into this category.

Dave
GW7ONS

Julesomega · 16th Aug 2020, 6:27 pm

The measurements seem to be made at AC with 25 or 250mV emf. There may also be a polarised output of about 2.5mV emf but I'll not pursue it further as it can go to GMB who can make better use of it

Bazz4CQJ · 16th Aug 2020, 8:41 pm

As commented above, you only have 'half' of the device; it needs a cell to immerse in the liquid, and to work with your particular meter it needs to have the correct 'cell constant' which I think is the area of the active electrodes divided by the distance between them.

You'd usually expect the cell to be fed with an AC voltage; with DC, polarisation will occur so the conductivity will appear to fall the longer the test continues.

B

16th Aug 2020, 4:25 pm	#1
Julesomega Nonode Join Date: Dec 2012 Location: Stockport, Greater Manchester, UK. Posts: 2,117	Uses of conductivity meter? I bought an interesting-looking meter off eBay under the same misapprehension as the seller, who listed it as a "Siemens meter" It is indeed, but was made by WPA of Linton, Cambridge, model CM35 Conductivity Meter It covers 10^-6 to 30^-2 Siemens. Not having any known conductors to hand I tried it with some resistors and it is reasonably accurate over this resistance range. My mind turns to jelly trying to convert between units, so I'm wondering who would want a meter to measure over this rather limited range instead of just picking up a multimeter? It is rather nice that the meter doesn't deflect on O/C, but do you really need a four-terminal measurement to measure down to only 33Ω? Attached Thumbnails __________________ - Julian It's good here

16th Aug 2020, 4:44 pm	#2
Sinewave Octode Join Date: May 2016 Location: Oxfordshire/Bucks borders, UK. Posts: 1,604	Re: Uses of conductivity meter? It's for meausring water/solution purity. Used in the process/plant industry. Last time I saw one was in the print industry for measuring purity for litho web offset press fount solution. __________________ Avometer, vintage Fluke and Marconi collector. Also interested in vintage Yaesu and KW.

16th Aug 2020, 5:06 pm	#3
GMB Dekatron Join Date: Aug 2003 Location: near Reading (and sometimes Torquay) Posts: 3,099	Re: Uses of conductivity meter? Do they measure with DC or AC? I was just about to build one, or at least something rather like it. I had assumed you had to use AC but if it is a 4-terminal device I guess that would be another way to go.

16th Aug 2020, 5:25 pm	#4
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,901	Re: Uses of conductivity meter? Conductivity meters often go with some sort of test cell. Some are for measuring liquids/pastes, others for powders. Some test cells ate flat surfaces for measuring the conductivity of antistatic surfaces. Sometimes used in agriculture for measuring the water content of grains to determine how much drying is needed. These versions may have a scale intended for the purpose and the specific test cell. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

16th Aug 2020, 5:33 pm	#5
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,604	Re: Uses of conductivity meter? Used similar meters in de-ionised water heat exchangers that were used to cool High Voltage equipment. Older non SI unit name for Siemens (Conductance) was the Mho.

16th Aug 2020, 5:42 pm	#6
ParcGwyn Hexode Join Date: Jun 2005 Location: Aberystwyth, Wales, UK. Posts: 358	Re: Uses of conductivity meter? They used to be used for checking moisture levels in wheat. the wheat was ground up and placed in cell that was compressed using a G-clamp. The more moisture the higher the conductivity and the lower the price. Not sure if your instrument falls into this category. Dave GW7ONS

16th Aug 2020, 6:27 pm	#7
Julesomega Nonode Join Date: Dec 2012 Location: Stockport, Greater Manchester, UK. Posts: 2,117	Re: Uses of conductivity meter? The measurements seem to be made at AC with 25 or 250mV emf. There may also be a polarised output of about 2.5mV emf but I'll not pursue it further as it can go to GMB who can make better use of it __________________ - Julian It's good here

16th Aug 2020, 8:41 pm	#8
Bazz4CQJ Dekatron Join Date: Apr 2009 Location: Oxfordshire, UK. Posts: 4,935	Re: Uses of conductivity meter? As commented above, you only have 'half' of the device; it needs a cell to immerse in the liquid, and to work with your particular meter it needs to have the correct 'cell constant' which I think is the area of the active electrodes divided by the distance between them. You'd usually expect the cell to be fed with an AC voltage; with DC, polarisation will occur so the conductivity will appear to fall the longer the test continues. B __________________ Saturn V had 6 million pounds of fuel. It would take thirty thousand strong men to lift it an inch.